Showing papers on "Magnetic structure published in 2015"

Magnetic structure of the quasi-two-dimensional antiferromagnet NiPS3

Abstract: The magnetic structure of the quasi-two-dimensional antiferromagnet NiPS3 has been determined by magnetometry and a variety of neutron diffraction techniques. The experiments show that the samples must be carefully handled, as gluing influences the magnetometry measurements while preferred orientation complicates the interpretation of powder diffraction measurements. Our global set of consistent measurements show numerous departures from previously published results. We show that the compound adopts a k = [010] antiferromagnetic structure with the moment directions mostly along the a axis, and that the paramagnetic susceptibility is isotropic. The critical behavior was also investigated through the temperature dependence of the magnetic Bragg peaks below the Neel temperature.

Ferromagnetic Order, Strong Magnetocrystalline Anisotropy, and Magnetocaloric Effect in the Layered Telluride Fe3−δGeTe2

Abstract: The ternary transition-metal compound Fe3−δGeTe2 is formed for 0 < δ < 0.3. X-ray diffraction and Mossbauer spectroscopy reveal its layered crystal structure with occasional Fe vacancies in the Fe2 site, whereas no Fe atoms occupy the interlayer space, so that only van der Waals interactions exist between adjacent layers. We explore magnetic behavior and ensuing functional properties of Fe2.9GeTe2 via neutron diffraction, thermodynamic and transport measurements, Mossbauer spectroscopy, and electronic structure calculations. Below TC = 225 K, Fe2.9GeTe2 is ferromagnetically ordered with the magnetic moments of 1.95(5) and 1.56(4) μB at T = 1.5 K, both directed along c, which is the magnetic easy axis. Electronic structure calculations confirm this magnetic structure and reveal a remarkably high easy-axis anisotropy of 4.2 meV/f.u. Mossbauer spectra reveal the magnetic ordering too, although a drastic influence of Fe vacancies on quadrupolar splittings and local magnetic fields has been observed. A moderat...

Magnetic Structure and Ordering of Multiferroic Hexagonal LuFeO 3

Abstract: We report on the magnetic structure and ordering of hexagonal ${\mathrm{LuFeO}}_{3}$ films of variable thickness grown by molecular-beam epitaxy on YSZ (111) and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ (0001) substrates. These crystalline films exhibit long-range structural uniformity dominated by the polar $P{6}_{3}cm$ phase, which is responsible for the paraelectric to ferroelectric transition that occurs above 1000 K. Using bulk magnetometry and neutron diffraction, we find that the system orders into a ferromagnetically canted antiferromagnetic state via a single transition below 155 K regardless of film thickness, which is substantially lower than that previously reported in hexagonal ${\mathrm{LuFeO}}_{3}$ films. The symmetry of the magnetic structure in the ferroelectric state implies that this material is a strong candidate for linear magnetoelectric coupling and control of the ferromagnetic moment directly by an electric field.

Study of the crystalline and magnetic structures of BaFe11.4Al0.6O19 in a wide temperature range

Abstract: The structural study of barium hexaferrite (BaFe12O19) whose iron ions are partly replaced by diamagnetic aluminum ions (x = 06) is carried out with the help of the neutron-diffraction method Experimental data are acquired in a wide temperature range of 4–730 K via a high-resolution diffractometer, which makes it possible to obtain both precise information on changes in crystalline and magnetic structures and behavioral data on the sample microstructure BaFe114Al06O19 retains the magnetoplumbite structure in the whole temperature range and exhibits the “invar effect,” according to which its bulk thermal-expansion coefficient is close to zero at low temperatures (<150 K) The magnetic structure is defined by the Gorter model with magnetic moments oriented along the hexagonal axis For all nonequivalent crystallographic positions, the magnetic moment of F3+ is close to 4 μB at 4 K The total magnetic moment per formula unit is 156 μB, ie, less than the nominal value of 20 μB This is caused by the fact that diamagnetic Al ions are included in the composition Crystallite microstrains increase insignificantly with temperature due to increasing influence of the magnetic subsystem

Antiferromagnetic structure in tetragonal CuMnAs thin films.

Abstract: Tetragonal CuMnAs is an antiferromagnetic material with favourable properties for applications in spintronics. Using a combination of neutron diffraction and x-ray magnetic linear dichroism, we determine the spin axis and magnetic structure in tetragonal CuMnAs, and reveal the presence of an interfacial uniaxial magnetic anisotropy. From the temperature-dependence of the neutron diffraction intensities, the Neel temperature is shown to be (480 ± 5) K. Ab initio calculations indicate a weak anisotropy in the (ab) plane for bulk crystals, with a large anisotropy energy barrier between in-plane and perpendicular-to-plane directions.

Investigation of the crystal and magnetic structures of BaFe 12 - x Al x O 19 solid solutions ( x = 0.1‒1.2)

Abstract: The structure of barium ferrite BaFe12 - x Al x O19 solid solutions (x = 0.1‒1.2) with iron partially replaced with diamagnetic aluminum ions has been studied by neutron diffraction. Experimental data have been collected at room temperature on a high-resolution diffractometer, which yielded precise information about the changes in the crystal and magnetic structures and data on the behavior of the sample microstructure. Barium hexaferrite retains a magnetoplumbite structure in the entire range of aluminum concentrations under study, and its magnetic structure is described within the Gorter model, with moments orientated along the hexagonal axis. The total magnetic moment per formula unit decreases while diamagnetic aluminum ions substitute for iron ions. Microstrains in crystallites increase with an increase in the diamagnetic ion concentration, which is related to the difference in the ionic radii of iron and aluminum ions.

Magnetic structure and crystal-field states of the pyrochlore antiferromagnet Nd 2 Zr 2 O 7

Abstract: In this paper, we present synchrotron x-ray diffraction, neutron powder diffraction, and time-of-flight inelastic neutron scattering measurements on the rare earth pyrochlore oxide Nd2Zr2O7 to study the ordered state magnetic structure and cystal-field states. The structural characterization by high-resolution synchrotron x-ray diffraction confirms that the pyrochlore structure has no detectable O vacancies or Nd/Zr site mixing. The neutron diffraction reveals long-range all-in/all-out antiferromagnetic order below TN≈0.4 K with propagation vector k = (0 0 0) and an ordered moment of 1.26(2) μB/Nd at 0.1 K. The ordered moment is much smaller than the estimated moment of 2.65μB/Nd for the local Ising ground state of Nd3+ (J=9/2) suggesting that the ordering is partially suppressed by quantum fluctuations. The inelastic neutron scattering experiment further confirms the Ising anisotropic ground state of Nd3+ and also reveals its dipolar-octupolar character which possibly induces the quantum fluctuation. Lastly, the crystal-field level scheme and ground state wave function have been determined.

Magnetic anisotropy of single-crystalline Mn3Sn in triangular and helix-phase states

Abstract: The magnetic structure and phase transitions in single-crystalline Mn3Sn with the D019 structure have been investigated by torque measurements and magnetic measurements. The results show that a triangular antiferromagnet (AFM) and a weak ferromagnet coexist above 270 K. At these temperatures, the torque measurements indicate a six-fold symmetry and another two-fold symmetry of the magnetic anisotropy. The [112¯0] direction is the lowest-energy state in the triangular configuration. At low temperatures, two AFM helix-phase states are found in an applied magnetic field (H): In one of them, the [112¯0] direction is the easiest deviation direction from the AFM state. In the other, it is the [0001] direction. The variation of the two states with temperature (T) and magnetic field is presented in an H-T phase diagram.

Invar-like Behavior of Antiperovskite Mn3+xNi1–xN Compounds

Abstract: The antiperovskite Mn3+xNi1–xN compounds have been synthesized and characterized by a variety of experimental techniques. After Mn doping at the Ni site, both ferromagnetic characteristics and an Invar-like effect were observed in the antiferromagnetic host material. The observed Invar-like behavior was assumed to be related to the characteristic magnetic structure induced by the doping. Neutron diffraction results prove that the Mn doping stabilizes the special Γ5g antiferromagnetic phase with strong spin–lattice coupling that can be tuned to achieve Invar-like behavior. The magnetovolume effect (MVE) and significant correlation between spin and lattice were confirmed for the Γ5g magnetic phase by the first-principles calculations. Moreover, Mn 3d electrons were revealed to be the key factor for the MVE from the calculations. Our study presents a new mechanism for precisely controlling the zero thermal expansion of a single compound by achieving the special Γ5g magnetic phase of Mn atoms.

Magnetic ordering and anisotropy in heavy atom radicals.

Abstract: Recent developments in stable radical chemistry have afforded “heavy atom” radicals, neutral open-shell (S = 1/2) molecular species containing heavy p-block elements (S, Se), which display solid-state magnetic properties once considered exclusive to conventional metal-based magnets. These highly spin-delocalized radicals do not associate in the solid state and yet display extensive networks of close intermolecular interactions. Spin density on the heavy atoms allows for increased isotropic and spin–orbit mediated anisotropic exchange effects. Structural variations induced by chemical modification and physical pressure, coupled with ab-initio methods to estimate exchange energies, have facilitated the development of predictive structure/property relationships. These results, coupled with detailed theoretical analyses and magnetic resonance spectroscopic measurements, have provided insight into the magnetic structure of ferromagnetic and spin-canted antiferromagnetic ordered materials as well as an understa...

Evolution of the magnetic structure with chemical composition in spinel iron oxide nanoparticles

Abstract: Magnetic properties of iron oxide nanoparticles with spinel structure are strictly related to a complex interplay between cationic distribution and the presence of a non-collinear spin structure (spin canting). With the aim to gain better insight into the effect of the magnetic structure on magnetic properties, in this paper we investigated a family of small crystalline ferrite nanoparticles of the formula CoxNi1−xFe2O4 (0 ≤ x ≤ 1) having equal size (≈4.5 nm) and spherical-like shape. The field dependence of magnetization at low temperatures indicated a clear increase of magnetocrystalline anisotropy and saturation magnetization (higher than the bulk value for CoFe2O4: ∼130 A m2 kg−1) with the increase of cobalt content. The magnetic structure of nanoparticles has been investigated by Mossbauer spectroscopy under an intense magnetic field (8 T) at a low temperature (10 K). The magnetic properties have been explained in terms of an evolution of the magnetic structure with the increase of cobalt content. In addition a direct correlation between cationic distribution and spin canting has been proposed, explaining the presence of a noncollinear spin structure in terms of superexchange interaction energy produced by the average cationic distribution and vacancies in the spinel structure.

High pressure floating zone growth and structural properties of ferrimagnetic quantum paraelectric BaFe12O19

Abstract: High quality single crystals of BaFe12O19 were grown using the floating zone technique in 100 atm of flowing oxygen. Single crystal neutron diffraction was used to determine the nuclear and magnetic structures of BaFe12O19 at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe3+ ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90 to 300 K. The inverse dielectric permittivity, 1/e, along the c-axis shows a T2 temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in 1/e. These features resemble those of classic quantum paraelectrics such as SrTiO3. The presence of the upturn in 1/e indicates that BaF...

Structure Symmetry Determination and Magnetic Evolution in Sr 2 Ir 1− x Rh x O 4

Abstract: We use single-crystal neutron diffraction to determine the crystal structure symmetry and to study the magnetic evolution in the rhodium doped iridates Sr2Ir1–xRhxO4 (0 ≤ x ≤ 0.16). Throughout this doping range, the crystal structure retains a tetragonal symmetry (space group I41/a) with two distinct magnetic Ir sites in the unit cell forming staggered IrO6 rotation. Upon Rh doping, the magnetic order is suppressed and the magnetic moment of Ir4+ is reduced from 0.21 μB/Ir for x = 0 to 0.18 μB/Ir for x = 0.12. As a result, the magnetic structure at x = 0.12 is different from that of the parent compound while the moments remain in the basal plane.

Evidence for magneto-electric and spin–lattice coupling in PbFe0.5Nb0.5O3 through structural and magneto-electric studies

Abstract: Neutron diffraction (ND) studies were carried out on polycrystalline single-phase multiferroic Pb(Fe0.5Nb0.5)O3 (PFN) in the temperature range of 290–2 K to understand the structural and magnetic properties as a function of temperature. ND data were refined using the Rietveld refinement method for both crystallographic and magnetic structures. The structure at room temperature was found to be monoclinic, in Cm space group. No structural transition was observed till 2 K. At low temperatures (i.e., from T < T N; T N = 155 K), an additional peak appears at scattering vector, Q = 1.35 A−1, indicating the onset of antiferromagnetic ordering. The magnetic structure was found to be commensurate with the crystallographic structure and could be refined using the propagation vector, k = [0.125, 0.5, and 0.5]. Magnetization, ferroelectric P–E loops, and dielectric measurements on PFN reveal a strong anomaly at the antiferromagnetic transition temperature (T N) indicating the magneto-electric coupling. The refined temperature-dependent structural parameters such as unit cell volume and monoclinic distortion angle (β) reveal pronounced anomalies at the magnetic ordering temperature (T N), which indicates strong spin–lattice coupling. An anomaly in lattice volume was observed with a small negative thermal expansion below and a large thermal expansion above the T N, respectively. It shows the occurrence of isostructural phase transition accompanying the magnetic ordering below T N ~155 K, leading to significant change in ionic polarization, octahedral tilt angle, and lattice strain around T N. We have used refined atomic positional coordinates from the nuclear and magnetic structures, to obtain ionic polarization. These detailed studies confirm the magneto-electric and spin–lattice coupling in PFN across T N.

Anomalous magnetic structure and spin dynamics in magnetoelectric LiFePO4

Abstract: We report significant details of the magnetic structure and spin dynamics of ${\mathrm{LiFePO}}_{4}$ obtained by single-crystal neutron scattering. Our results confirm a previously reported collinear rotation of the spins away from the principal $b$ axis, and they determine that the rotation is toward the $a$ axis. In addition, we find a significant spin-canting component along $c$. The possible causes of these components are discussed, and their significance for the magnetoelectric effect is analyzed. Inelastic neutron scattering along the three principal directions reveals a highly anisotropic hard plane consistent with earlier susceptibility measurements. Using a spin Hamiltonian, we show that the spin dimensionality is intermediate between $XY$- and Ising-like, with an easy $b$ axis and a hard $c$ axis. It is shown that both next-nearest neighbor exchange couplings in the $bc$ plane are in competition with the strongest nearest neighbor coupling.

Integer and fractional quantum anomalous Hall effect in a strip of stripes model

Abstract: We study the quantum anomalous Hall effect in a strip of stripes model coupled to a magnetic skyrmion texture with zero total magnetization and in the presence of strong electron-electron interactions. A helical magnetization along the stripes and a spin-selective coupling between the stripes gives rise to a bulk gap and chiral edge modes. Depending on the ratio between the period of the magnetic structure and the Fermi wavelength, the system can exhibit the integer or fractional quantum anomalous Hall effect. In the fractional regime, the quasiparticles have fractional charges and nontrivial Abelian braid statistics.

Magnetic compensation, field-dependent magnetization reversal, and complex magnetic ordering in Co 2 TiO 4

Abstract: The complex nature of magnetic ordering in the spinel Co2TiO4 is investigated by analyzing the temperature and magnetic field dependence of its magnetization (M), specific heat (C-p), and ac magnetic susceptibilities chi' and chi ''. X-ray diffraction of the sample synthesized by the solid-state reaction route confirmed the spinel structure whereas x-ray photoelectron spectroscopy shows its electronic structure to be Co2TiO4 = [Co2+][Co3+ Ti3+]O-4. From analysis of the temperature dependence of the dc paramagnetic susceptibility, the magnetic moments mu(A) = 3.87 mu(B) and mu(B) = 5.19 mu B on the A and B sites are determined with mu(B) in turn yielding mu(Ti3+) = 1.73 mu(B) and mu(Co3+) = 4.89 mu(B). Analysis of the dc and ac susceptibilities combined with the weak anomalies observed in the C-p vs T data shows the existence of a quasi-long-range ferrimagnetic state below T-N similar to 47.8K and a compensation temperature T-comp similar to 32 K, the latter characterized by sign reversal of magnetization with its magnitude depending on the applied magnetic field and the cooling protocol. Analysis of the temperature dependence of M (field cooled) and M (zero field cooled) data and the hysteresis loop parameters is interpreted in terms of large spin clusters. These results in Co2TiO4, significantly different from those reported recently in isostructural Co2SnO4 = [Co2+][Co2+ Sn4+]O-4, warrant further investigations of its magnetic structure using neutron diffraction.

Phonon and magnetic structure in δ-plutonium from density-functional theory

Abstract: We present phonon properties of plutonium metal obtained from a combination of density-functional-theory (DFT) electronic structure and the recently developed compressive sensing lattice dynamics (CSLD). The CSLD model is here trained on DFT total energies of several hundreds of quasi-random atomic configurations for best possible accuracy of the phonon properties. The calculated phonon dispersions compare better with experiment than earlier results obtained from dynamical mean-field theory. The density-functional model of the electronic structure consists of disordered magnetic moments with all relativistic effects and explicit orbital-orbital correlations. The magnetic disorder is approximated in two ways: (i) a special quasi-random structure and (ii) the disordered-local-moment method within the coherent potential approximation. Magnetism in plutonium has been debated intensely, but the present magnetic approach for plutonium is validated by the close agreement between the predicted magnetic form factor and that of recent neutron-scattering experiments.

Discovery of a strongly phase-variable spectral feature in the isolated neutron star RX J0720.4–3125

Abstract: We present the discovery of a strongly phase-variable absorption feature in the X-ray spectrum of the nearby, thermally emitting, isolated neutron star RX J0720.4-3125. The absorption line was detected performing detailed phase-resolved spectroscopy in 20 XMM-Newton observations, covering the period 2000 May-2012 September. The feature has an energy of ~750 eV, an equivalent width of ~30 eV, and it is significantly detected for only ~20% of the pulsar rotation. The absorption feature appears to be stable over the timespan covered by the observations. Given its strong dependence on the pulsar rotational phase and its narrow width, a plausible interpretation is in terms of resonant proton cyclotron absorption/scattering in a confined magnetic structure very close to the neutron star surface. The inferred field in such a magnetic loop is Bloop ∼ 2 x times 10 G, a factor of ~7 higher than the surface dipolar magnetic field.

Magnetic proximity effect and interlayer exchange coupling of ferromagnetic/topological insulator/ferromagnetic trilayer

Abstract: The magnetic proximity effect between the topological insulator (TI) and ferromagnetic insulator (FMI) is considered to have great potential in spintronics. However, a complete determination of interfacial magnetic structure has been highly challenging. We theoretically investigate the interlayer exchange coupling of two FMIs separated by a TI thin film, and show that the particular electronic states of the TI contributing to the proximity effect can be directly identified through the coupling behavior between two FMIs, together with a tunability of the coupling constant. Such an FMI/TI/FMI structure not only serves as a platform to clarify the magnetic structure of the FMI/TI interface, but also provides insights in designing the magnetic storage devices with ultrafast response.

Magnetic structure and Magnetic transport Properties of Graphene Nanoribbons With Sawtooth Zigzag Edges

Abstract: The magnetic structure and magnetic transport properties of hydrogen-passivated sawtooth zigzag-edge graphene nanoribbons (STGNRs) are investigated theoretically. It is found that all-sized ground-state STGNRs are ferromagnetic and always feature magnetic semiconductor properties, whose spin splitting energy gap Eg changes periodically with the width of STGNRs. More importantly, for the STGNR based device, the dual spin-filtering effect with the perfect (100%) spin polarization and high-performance dual spin diode effect with a rectification ratio about 1010 can be predicted. Particularly, a highly effective spin-valve device is likely to be realized, which displays a giant magnetoresistace (MR) approaching 1010%, which is three orders magnitude higher than the value predicted based on the zigzag graphene nanoribbons and six orders magnitude higher than previously reported experimental values for the MgO tunnel junction. Our findings suggest that STGNRs might hold a significant promise for developing spintronic devices.

High pressure floating zone growth and structural properties of ferrimagnetic quantum paraelectric BaFe$_{12}$O$_{19}$

Abstract: High quality single crystals of BaFe$_{12}$O$_{19}$ were grown using the floating zone technique in flowing oxygen pressurized to 100 atm. Single crystal neutron diffraction was used to determine the nuclear and magnetic structure of BaFe$_{12}$O$_{19}$ at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe$^{3+}$ ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90-300 K. The inverse dielectric permittivity, $1/\varepsilon$, along the c-axis shows a $T^2$ temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in $1/\varepsilon$. These features resemble those of classic quantum paraelectrics such as SrTiO$_3$. The presence of the upturn in $1/\varepsilon$ indicates that BaFe$_{12}$O$_{19}$ is a critical quantum paraelectric system with Fe$^{3+}$ ions involved in both magnetic and electric dipole formation.

Mapping the magnetic and crystal structure in cobalt nanowires

Abstract: Using off-axis electron holography under Lorentz microscopy conditions to experimentally determine the magnetization distribution in individual cobalt (Co) nanowires, and scanning precession-electron diffraction to obtain their crystalline orientation phase map, allowed us to directly visualize with high accuracy the effect of crystallographic texture on the magnetization of nanowires. The influence of grain boundaries and disorientations on the magnetic structure is correlated on the basis of micromagnetic analysis in order to establish the detailed relationship between magnetic and crystalline structure. This approach demonstrates the applicability of the method employed and provides further understanding on the effect of crystalline structure on magnetic properties at the nanometric scale.

Shape-dependent exchange bias effect in magnetic nanoparticles with core-shell morphology

Abstract: We study the low-temperature isothermal magnetic hysteresis of cubical and spherical nanoparticles with ferromagnetic-core/antiferromagnetic-shell morphology, in order to elucidate the sensitivity of the exchange bias effect to the shape of the particles and the structural imperfections at the core-shell interface. We model the magnetic structure using a classical Heisenberg Hamiltonian with uniaxial anisotropy and simulate the hysteresis loop using the metropolis Monte Carlo algorithm. For nanoparticles with geometrically sharp interfaces, we find that cubes exhibit a higher coercivity and lower exchange bias field than spheres of the same size. With increasing interface roughness, the shape dependence of the characteristic fields gradually decays, and eventually, the distinction between cubical and spherical particles is lost for moderately rough interfaces. The sensitivity of the exchange bias field to the microstructural details of the interface is quantified by a scaling factor ($b$) relating the bias field to the net moment of the antiferromagnetic shell $({H}_{\mathrm{eb}}=b{M}_{\mathrm{AF}}+{H}_{o})$. Cubical particles exhibit a lower sensitivity to the dispersed values of the net interfacial moment.

Synthesis, structures and magnetic properties of the dimorphic Mn2CrSbO6 oxide

Abstract: The perovskite polymorph of Mn(2)CrSbO(6) compound has been synthesized at 8 GPa and 1473 K. It crystallizes in the monoclinic P21/n space group with cell parameters a = 5.2180 (2) A, b = 5.3710(2) A, c = 7.5874(1) A and β = 90.36(1)°. Magnetic susceptibility and magnetization measurements show the simultaneous antiferromagnetic ordering of Mn(2+) and Cr(3+) sublattices below TN = 55 K with a small canting. Low temperature powder neutron diffraction reveals a commensurate magnetic structure with spins confined to the ac-plane and a propagation vector κ = [1/2 0 1/2]. The thermal treatment of this compound induces an irreversible phase transition to the ilmenite polymorph, which has been isolated at 973 K and crystallizes in R3[combining macron] space group with cell parameters a = 5.2084 (4) A and c = 14.4000 (11) A. Magnetic susceptibility, magnetization and powder neutron diffraction data confirm the antiferromagnetic helical ordering of spins in an incommensurate magnetic structure with κ = [00 0.46] below 60 K, and the temperature dependence of the propagation vector up to κ = [00 0.54] at about 10 K.

Spin orientation, structure, morphology, and magnetic properties of hematite nanoparticles

Abstract: Monodisperse hematite (α-Fe2O3) nanoparticles were synthesized by forced hydrolysis of acidic Fe3+ solution. Rietveld analysis was applied to the X-ray powder diffraction data to refine the lattice constants and atomic positions. The lattice constants for a hexagonal unit cell were determined to be a ∼ 0.50327 and c ∼ 1.37521 nm. High resolution transmission electron microscopy was employed to study the morphology of the particles. Atomic scale micrographs and diffraction patterns from several zone axes were obtained. These reveal the high degree of crystallinity of the particles. A series of observations made on the particles by tilting them through a range of ±45° revealed the particles to be micaceous with stacking of platelets with well defined crystallographic orientations. The Morin transition in these nanoparticles was found to occur at 210 K, which is lower temperature than 263 K of bulk hematite. It was ascertained from the previous Mossbauer studies that the spin orientation for nano-sized hemat...

Two-Dimensional Charge Disproportionation of the Unusual High Valence State Fe4+ in a Layered Double Perovskite

Abstract: The crystal and magnetic structures of charge-disproportionated Ca2FeMnO6 were analyzed by neutron powder diffraction. Ca2FeMnO6 is a layered double perovskite oxide with a two-dimensional arrangement of Mn4+ and unusual high valence Fe4+ at room temperature. When cooled, the compound shows charge disproportionation followed by magnetic transition. Around 200 K, the Fe4+ shows the charge disproportionation to Fe3+ and Fe5+, which are ordered in a checkerboard pattern in the two-dimensional FeO6 octahedral layers. The magnetic transition occurs at 95 K, which is much lower than the charge disproportionation temperature. The magnetic structure is commensurate but noncollinear, and the antiferromagnetic coupling of Fe3+ and Fe5+ spins in the FeO6 octahedral layers gives the ferrimagnetic moments. The unique magnetic structure is described as a result of two-dimensional localization of the ligand holes with effective spins.